Modern radio frequency (RF) communication systems employ advanced signal modulation techniques to modulate the carrier frequency by a digital baseband signal. These techniques include, for example, Phase-Shift Keying (PSK), Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), Frequency Shift Keying (FSK) and Minimum Shift Keying (MSK). The use of digital modulation techniques versus analog modulation techniques leads to improved system performance, lower cost, higher reliability, greater capacity and increased security. However, these improvements come at the price of increased system complexity, particularly in the design of radio transceivers.
The use of digital modulation techniques for transmitting data requires that subsequent demodulation of the received data take place in the digital domain. This allows the use of sophisticated digital signal processing (DSP) techniques to improve data reception in the presence of many real-world imperfections such as multipath interference, intersymbol interference, fading, and the like.
For data reception, the in-coming RF signal is received at the antenna, amplified, translated to a lower frequency, filtered and then converted from the analog domain to the digital domain for further processing and eventual demodulation. The receive path is split into two parallel paths during the frequency translation to baseband frequency. This step is typically carried out as a quadrature down-conversion resulting in both an in-phase (commonly referred to as channel I) and a quadrature (commonly referred to as channel Q) component of the in-coming receive data. Channels I and Q are subsequently processed simultaneously in two parallel, well-matched signal paths. By processing the signals in two parallel, well matched signal paths, unwanted by-product of the frequency translation or down mixing can be substantially cancelled when the two channels are re-combined in the digital domain. However, the effectiveness of this cancellation scheme is fundamentally limited by the amount of gain and phase mismatch in the two parallel signal paths I and Q, also referred to as I-Q mismatch.
Depending on the receiver architecture, e.g., heterodyne, homodyne, or image-reject, I-Q mismatch can substantially affect system performance. The amount of tolerable I-Q mismatch varies with the architecture. Typically, image-reject and homodyne systems are more sensitive than heterodyne systems. In any case, I-Q mismatch leads to incomplete cancellation of the image frequency when the channels are recombined in the digital domain. Incomplete cancellation of the image frequency degrades the signal-to-noise-ratio (SNR) in the desired frequency band and results in sub-optimal receiver performance.
The sources of mismatch between channels I and Q are many. For example, the gain and phase mismatch of the local oscillator contributes a mismatch term. In addition, each stage in the two parallel signal paths contributes to the overall mismatch. While some sources of mismatch are static, others can vary over time thereby making it more difficult to track and compensate for the mismatch. As an example, it may be particularly difficult to compensate for mismatch caused by ADCs in which the reference voltage mismatch changes as a function of operating temperature.